What Goes Up Does Come Down

If Cassini's plutonium payload comes raining down on Earth, it won't be the first time.

A space probe, its instruments powered by plutonium, screws up and comes falling back to earth, dumping its radioactive cargo. This isn't just the worst-case scenario for NASA's Cassini mission, it's what happened last November, and at least eight times before that.

Russia's ill-fated Mars '96 probe was on its way to the red planet when it failed to break out of Earth's gravity. The craft and its four plutonium-filled capsules, containing a half-pound of the deadliest substance known to science, came plummeting back to Earth -- and may still be lying somewhere in a Bolivian salt plain.

The Mars '96 accident spurred a flurry of media attention when trackers predicted an Australian landing for the broken probe. But concern died down when the craft was reported to have plunked into the Pacific Ocean, 23 miles off the coast of Chile -- even though Chilean environmentalists warned of the possibility of polluted fishing waters. As a headline in the Nov. 18 Washington Post read: "Errant Russian Spacecraft Crashes Harmlessly After Scaring Australia."

By the time the news came a month later that the probe in fact had fallen onto a remote area of Bolivia near the Chilean border, media coverage had petered out. The discovery made headlines in South America, but received scant coverage in the U.S. press. The small canisters of plutonium which were to power the probe still have not been recovered, but air tests conducted by Chilean authorities have detected no radiation in the area.

Otto Raabe, president of the Health Physics Society -- a group of experts on radiation safety -- says concern about plutonium 238 from space-bound nuclear batteries, known as radioisotope thermoelectric generators (RTGs), is overblown. "It's a potentially hazardous material," he says. "But it's hard to be exposed to it."

Some who followed last fall's accident -- like journalism professor and Cassini opponent Karl Grossman, who has written extensively on nuclear issues -- feel that efforts to find the wreckage were hampered by a two-week delay by the U.S. Space Command in announcing the true landing site.

"There was all kinds of attention when it appeared the craft was going to fall on Australia, where people are white," Grossman says. "Then when it turned out it fell on Latin America, the U.S. media and the American government were nowhere to be found."

The U.S. Space Command, called in by Russia for its tracking capabilities, contends the delay had nothing to do with skin color -- and more to do with the usual bureaucracy and a bit of botched Russian-American communication. When the Russians enlisted U.S. help to find their errant probe, explains Spacecom spokesman David Knox, they failed to mention that the probe had already re-entered the atmosphere -- the day before.

Regardless of what caused the delay or whether scientists agree on the toxicity of radioactive fallout, the fact remains that Mars '96 was not an anomaly. At least eight other Russian and American missions have returned their nuclear cargo back to Earth:

COSMOS 1402, launched by the former U.S.S.R., re-entered the atmosphere Feb. 7, 1983, over the South Atlantic, bringing with it 68 pounds of uranium 235. The craft disintegrated in the upper atmosphere.

Another USSR-launched craft, COSMOS 954, returned Jan. 24, 1978, over Canada, carrying 68 pounds of uranium 235. Seventy-five percent of the load apparently burned up in the atmosphere, while the rest rained down on 40,000 square miles of unpopulated tundra. More than $10 million was spent by Canada, the United States, and Russia in cleaning up the area, which had been littered with thousands of radioactive balls, says Steven Aftergood, a senior research analyst for the Federation of American Scientists.

In April 1973, another U.S.S.R. mission, a RORSAT, released radiation from its reactor into the Pacific Ocean, north of Japan.

The well-known U.S. Apollo 13 mission brought back 5.5 pounds of plutonium 238 on April 14, 1970. The material is thought to have sunk somewhere in the South Pacific Ocean off Tonga, says NASA public affairs officer Donald Savage. "There was no release of radiation into the air," he says, "so it's most likely that it remained intact and is inert," too deep in the sea to be recovered.

In 1969, radiation was detected when two Soviet COSMOS lunar missions burned up in the atmosphere.

The U.S. NIMBUS B-1 re-entered the atmosphere May 18, 1968, and about 4 pounds of plutonium 238 landed in the Santa Barbara channel off California. According to Savage, the fuel was recovered and used in another mission.

On April 21, 1964, TRANSIT 5BN-3, a U.S. craft, dropped about 2 pounds of plutonium 238 over the Indian Ocean. The nuclear material vaporized and spread worldwide. "There were no identifiable health effects," Aftergood says, "but this added some fraction to our radiation background." The accident was enough to spur a redesign of the nuclear batteries which came to be known as RTGs -- which now await launch aboard the Cassini probe.